Heat transfer pipe

ABSTRACT

A heat transfer pipe for use in a heat exchanger such as air conditioner, freezer and boiler, wherein grooves are formed in the inner wall surface of the pipe, which are by far finer in size than the grooves that have been provided for the purpose of increasing the heat transfer area in general, and slanting relative to the axis of pipe, to thereby improve the heat transfer rate without increasing the pressure loss caused to the fluid flowing through the pipe.

FIELD OF THE INVENTION

This invention relates to a heat transfer pipe for use in a heatexchanger such as air conditioner, freezer and boiler.

DESCRIPTION OF THE PRIOR ART

Heat transfer pipes having the purpose of improving the heat transferrate between the heat transfer pipe and the fluid flowing through thepipe include a pipe provided therein with fins closely adhering to theinner wall thereof and a pipe provided in the inner wall thereof withgrooves. Both pipes are intended to increase the heat transfer area inthe pipes and expand turbulence of fluid in the pipes by providing finsor grooves, thereby improving the heat transfer rate per unit length ofthe heat transfer pipes. Accordingly, it is necessary that the height offins or the depth of grooves should reach or exceed a certain level.With the aforesaid arrangements, the heat transfer pipes have rendered ahigh level of resistance to the fluid flowing through the pipe, therebyunavoidably receiving a fairly large pressure loss.

Increased pressure loss requires a large pumping power and moreoverresults in varied condensation and evaporation temperatures and causeshampered performance of the heat exchanger or the operating system as awhole, whereby the adoption of the heat transfer pipes of the type hasbeen hindered.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a heat transfer pipehaving a high heat transfer rate. Another object of the presentinvention is to provide a heat transfer pipe with a low pressure loss.To accomplish the objects described, this invention is characterized inthat grooves are formed in the inner wall surface of the pipe, which areby far finer in size than the grooves that have been provided for thepurpose of increasing the heat transfer area on the inner wall surfaceof pipes in general, and slanting at an acute angle relative to the axisof the pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged view of a cross section of a heat transfer havingV-shaped grooves pipe according to the present invention, which issectioned by a plane perpendicular to the grooves;

FIG. 2 is an enlarged view of a cross section of a heat transfer pipeaccording to the present invention, which is sectioned by a planeincluding the axis of the pipe;

FIG. 3 is an enlarged view of another heat transfer having U-shapedgrooves pipe according to the present invention, which is sectioned by aplane perpendicular to the groove;

FIG. 4 is a diagram showing the relationship between the depth of grooveand the heat transfer rate;

FIG. 5 is a diagram showing the relationship between the depth of grooveand the ratio of pressure losses;

FIG. 6 is a diagram showing the relationship between the inclination ofgroove and the heat transfer rate and the relationship between theinclination of groove and the pressure loss;

FIG. 7 is a diagram showing the relationship between the difference intemperature and the heat flux;

FIG. 8 is a diagram showing the relationship between the flow rate ofrefrigerant and the pressure loss;

FIG. 9 is a diagram showing the relationship between the apex angle ofgroove which is V-shaped in section and the heat transfer rate; and

FIG. 10 is a diagram showing the relationship between the width ofgroove which is U-shaped in section and the heat transfer rate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is an enlarged view of a cross section of a heat transfer pipeaccording to the present invention, which is sectioned by a planeperpendicular to the groove. FIG. 2 is an enlarged view of a crosssection of a heat transfer pipe according to the present invention,which is sectioned by a plane including the axis of the pipe. Amultitude of grooves 2 which are V-shaped in section are provided in theinner wall surface of the heat transfer pipe 1. The depth h of grooves 2from the inner wall surface ranges from 0.02 to 0.2 mm. The intervalbetween a groove and the next groove, i.e., the pitch p ranges from 0.1to 0.5 mm. The apex angle γ ranges from 30° to 90°. Additionally, thegrooves 2 are formed in the inner wall surface of the heat transfer pipe1 in the spiral shape. More specifically, the inclination β relative tothe axis 3 of the heat transfer pipe 1 is given by:

    0° < β < 90° , 90° < β < 180°

FIG. 3 is an enlarged view of another heat transfer pipe according tothe present invention, which is sectioned by a plane perpendicular tothe groove. The grooves 2 are U-shaped in section. The depth h, pitch pand the inclination β of grooves 2 are identical with that in thepreceding embodiment.

FIG. 4 is a diagram showing the relationship between the depth h ofgroove and the heat transfer rate α, wherein the depth h is given as anabscissa and the heat transfer rate α of a heat transfer pipe providedwith the grooves as against the heat transfer rate α_(o) of a smoothpipe is given as an ordinate.

The conditions of this experiment were as follows:

    ______________________________________                                        Material of the heat transfer pipe                                                                   Copper                                                 Inner diameter d of the heat transfer                                         pipe                   11.2 mm                                                Depth h of the groove  gradually varied                                                              from 0.02 mm                                           Pitch p of the groove  0.5                                                    Inclination β of the groove                                                                     45°                                             Apex angle γ of the groove                                                                     60°                                             Refrigerant used       R-22                                                   Pressure of the boiling liquid                                                                       4 kg/cm.sup.2 G                                        Flow rate (weight) Gr of the boiling                                          liquid                 60 kg/hr                                               Heat flux q applied    500 Kcal/m.sup.2 hr                                    Average mass vapor quality -x                                                                        0.6                                                    ______________________________________                                    

As apparent from FIG. 4, the heat transfer pipe provided in the innerwall surface thereof with the grooves 2 shows a high heat transfer rate,when the depth h of the groove 2 ranges from 0.02 to 0.2 mm, said ratereaching three times as high as that of the smooth pipe at its maximum.Such a high heat transfer rate can be attributed to the fact that whenthe groove 2 has the values of 0.02 to 0.2 mm in depth h and of 0.1 to0.5 mm in pitch p, a boiling fluid passing through the heat transferpipe 1 receives a rotating force along the pipe wall, and flows in amanner that said boiling fluid forms a thin film which almost adheres tothe entire area of inner wall surface of heat transfer pipe 1 due tocapillarity, with the gaseous portion of the boiling fluid flows throughthe central portion of heat transfer pipe 1. Furthermore, the grooves 2serve as the centers of boiling since the grooves are so fine in size.

FIG. 5 is a diagram showing the relationship between the depth h ofgroove 2 and the pressure loss ΔP, wherein the depth h is given as anabscissa and the ratio between the pressure loss ΔP of heat transferpipe provided therein with the grooves 2 and the pressure loss ΔP_(o) ofsmooth pipe (ΔP/ΔP_(o)) is given as an ordinate.

The measurements of the pressure losses were made in parallel with themeasurements of the aforesaid heat transfer rate α under the conditionsidentical with the preceding embodiment.

As apparent from FIG. 5, the pressure loss is increased with increase ofthe depth h of the groove 2 in the manner of a curve of the secondorder. When the depth h of groove 2 is less than 0.2 mm, the pressureloss of a heat transfer pipe provided therein with grooves issubstantially equal to that of smooth pipe. In that case, the provisionof grooves 2 hardly contributes to the increase in pressure loss.

The equality of pressure losses between the heat transfer pipe providedwith the grooves 2 having a depth h less than 0.2 mm and the smooth pipecan be attributed to the fact that when the boiling fluid flows in amanner that the liquid is adhering to the inner wall surface of heattransfer pipe 1, said liquid covers and renders smoothness to thegrooves 2, and forms a free interface having less resistance than thatthe solid wall has in the case of the conventional heat transfer pipeprovided therein with deep grooves.

FIG. 6 is a diagram showing the relationship between the inclination βof groove 2 and the heat transfer rate α, wherein the inclination β ofgroove 2 is given as an abscissa and the heat transfer rate α is givenas an ordinate.

As the criterion in comparison, the heat transfer rate of smooth pipe isshown to the left in the drawing. Additionally, the conditions of thisexperiment were as follows:

    ______________________________________                                        Material of the heat transfer pipe                                                                   Aluminum                                               Inner diameter d of the heat transfer                                         pipe                   11.2 mm                                                Depth h of the groove  0.15 mm                                                Pitch p of the groove  0.5 mm                                                 Inclination β of the groove                                                                     gradually varied                                                              from 0°                                         Apex angle γ of the groove                                                                     90°                                             Refrigerant used       R-22                                                   Pressure of the boiling liquid                                                                       4 kg/cm.sup.2 G                                        Flow rate (weight) Gr of the boiling                                          liquid                 43 kg/hr                                               Heat flux q applied    18,300 Kcal/m.sup.2 hr                                 Average mass vapor quality -x                                                                        0.6                                                    ______________________________________                                    

Apparent from FIG. 6, the pressure loss ΔP is hardly affected by theinclination β of groove 2 and substantially constant. The heat transferrate α is greatly varied by the inclination β of groove 2. When theinclination β = 0°, i.e., the grooves 2 are parallel to the axis 3 ofthe heat transfer pipe 1, a value lower than that of smooth pipe isindicated, and the rise becomes sharper with increase of the inclinationβ. The maximum value is reached in the vicinity of the inclination βbeing 7°. The value decreases with rise of the inclination β from 7°,and gradually increases with rise of the inclination β from approx. 45°.

Then, the provision of grooves 2 on the inner wall surface of heattransfer pipe 1 increases area of the inner surface which is concernedwith heat transmission of the heat transfer pipe by approx. 35%, andlittle effect is found due to the difference of the inclination β ofgroove 2 in degree.

As described above, when area of the inner surface is increased,naturally the heat transfer rate is improved. Now, if assumption is madethat all the increased surface area is uniformly concerned with heattransmission, then the heat transfer rate is risen by 35% and can beindicated by a straight line A. Therefore, the inclination β indicatinga heat transfer rate higher than the value indicated by the straightline A is included within the range from 4° to 15°, which can be calledthe preferable range of inclination.

Said inclination range from 4° to 15° is regarded as the inclinationrange which is effective in rendering a large rotating force to theboiling liquid through the agency of the gas flowing through the centralportion of heat transfer pipe, thereby lifting the boiling liquid liableto gather in the lower portion of heat transfer pipe.

FIG. 7 shows the relationship between the heat flux q (Kcal/m² hr) andthe difference in temperature ΔT (° C.) (The difference in temperaturemeans the difference between the temperature of pipe wall of heattransfer pipe 1 and the saturation temperature of boiling liquid.),wherein the difference in temperature ΔT (° C.) is given as an abscissaand the heat flux q (Kcal/m² hr) is given as an ordinate.

A curve q₁ indicates the heat flux of heat transfer pipe according tothe present invention and q₀ the heat flux of smooth pipe. Theconditions of this experiment were as follows:

    ______________________________________                                        Material of the heat transfer pipe                                                                   Copper                                                 Inner diameter d of the heat transfer                                         pipe                   11.2 mm                                                Depth h of the groove  0.1 mm                                                 Pitch p of the groove  0.5 mm                                                 Inclination β of the groove                                                                     45°                                             Apex angle γ of the groove                                                                     60°                                             Refrigerant used       R-22                                                   Pressure of the boiling liquid                                                                       4 kg/cm.sup.2 G                                        Flow rate (weight) Gr of the boiling                                          liquid                 43 kg/hr                                               Average mass vapor quality -x                                                                        0.6                                                    ______________________________________                                    

As apparent from FIG. 7, it is found that the heat transfer pipeaccording to the present invention has the heat flux superior to that ofthe smooth pipe over all range of differences in temperature. FIG. 8 isa diagram showing the relationship between the refrigerant flow rate Gr(kg/hr) and the pressure loss ΔP (kg/cm²) per meter of heat transferpipe, wherein the refrigerant flow rate Gr (kg/hr) is given as anabscissa and the pressure loss ΔP (kg/cm²) is given as an ordinate. Acurve ΔP₁ indicates the pressure loss of heat transfer pipe 1 accordingto the present invention and a curve ΔP_(o) the pressure loss of smoothpipe. The conditions of this experiment were as follows:

    ______________________________________                                        Material of the heat transfer pipe                                                                   Copper                                                 Inner diameter d of the heat transfer                                         pipe                   11.2 mm                                                Depth h of the groove  0.1 mm                                                 Pitch p of the groove  0.5 mm                                                 Inclination β of the groove                                                                     45°                                             Apex angle γ of the groove                                                                     60°                                             Refrigerant used       R-22                                                   Pressure of the boiling liquid                                                                       4 kg/cm.sup.2 G                                        Flow rate (weight) of the boiling                                             liquid Gr              43 kg/hr                                               Heat flux q applied    12,000 Kcal/m.sup.2 hr                                 Average mass vapor quality -x                                                                        0.6                                                    ______________________________________                                    

As apparent from said FIG. 8, it is found that the heat transfer pipeaccording to the present invention has the pressure loss somewhat lowerthan the smooth pipe over all range of the flow rates of refrigerantflowing through the heat transfer pipe.

FIG. 9 is a diagram showing the relationship between the variation ofapex angle of groove 2 and the heat transfer rate α in the case of thegrooves 2 being V-shaped in section, wherein the refrigerant flow rateGr (kg/hr) is given as an abscissa and the heat transfer rate α (Kcal/m²hr° C.) is given as an ordinate. Referring to said FIG. 9, a curve γ₃₀indicates the case where the apex angle γ is 30°, a curve γ₆₀ the casewhere the apex angle γ is 60°, a curve γ₉₀ the case where the apex angleγ is 90°, and a curve γ_(o) the case of smooth pipe used. The conditionsof the experiment were as follows:

    ______________________________________                                        Material of the heat transfer pipe                                                                   Copper                                                 Inner diameter d of the heat transfer                                         pipe                   11.2 mm                                                Depth h of the groove  0.2 mm                                                 Pitch p of the groove  0.5 mm                                                 Inclination β of the groove                                                                     84°                                             Refrigerant used       R-22                                                   Pressure of the boiling liquid                                                                       4 kg/cm.sup.2 G                                        Heat flux q applied    18,000 Kcal/m.sup.2 hr                                 Average mass vapor quality -x                                                                        0.6                                                    ______________________________________                                    

As apparent from said FIG. 9, when the grooves 2 are V-shaped insection, there is indicated that the sharper the apex angle is, thehigher the heat transfer rate is obtained. FIG. 10 is a diagram showingthe variation of the heat transfer rate α (Kcal/m² hr° C.) in accordancewith the variation of the width w of groove 2 in the case of the grooves2 being U-shaped in section, wherein the refrigerant flow rate Gr(kg/hr) is given an abscissa and the heat transfer rate α (Kcal/m² hr°C.) is given as an ordinate.

A curve W_(o) indicates the heat transfer rate in the case of smoothpipe used, a curve W₁ that in the case of the width w of groove 2 beingapprox. 0.9 mm, a curve W₂ that in the case of the width w of groove 2being approx. 0.5 mm, and a curve W₃ that in the case the width w ofgroove 2 being approx. 0.25 mm. The condition of this experiment were asfollows:

    ______________________________________                                        Material of the heat transfer pipe                                                                   Copper                                                 Inner diameter d of the heat transfer                                         pipe                   11.2 mm                                                Depth h of the groove  0.2 mm                                                 Inclination β of the groove                                                                     84°                                             Refrigerant used       R-22                                                   Pressure of the boiling liquid                                                                       4 kg/cm.sup.2 G                                        Heat flux q applied    18,000 Kcal/m.sup.2 hr                                 Average mass vapor quality -x                                                                        0.6                                                    ______________________________________                                    

As apparent from said FIG. 10, when the grooves 2 are U-shaped insection, the narrower the width w of groove 2 are, i.e., the smaller thepitch p of groove 2 are, the higher the heat transfer rate can beobtained.

What is claimed is:
 1. In a heat transfer pipe for forced convectionboiling or condensing the improvement comprising said heat transfer pipebeing formed on its inner wall surface with grooves having a depth fromthe wall surface to their bottoms in the range between 0.02 and 0.2millimeters, a pitch between the adjacent grooves in the range between0.1 and 0.5 millimeters, and an angle of inclination with respect to theaxis of the heat transfer pipe arranged between 4° and 15° or 165° and176°.
 2. A heat transfer pipe as claimed in claim 1, wherein the angleof inclination of said grooves with respect to the axis of the heattransfer pipe is 7°.
 3. A heat transfer pipe as defined in claim 1,wherein the grooves narrow from the inner wall surface to their bottoms.4. A heat transfer pipe as defined in claim 3, wherein the grooves areV-shaped in section.
 5. A heat transfer pipe as defined in claim 4,wherein the apex angle of groove is no more than 90°.
 6. A heat transferpipe as defined in claim 4, wherein the apex angle of groove ranges from30° to 60°.
 7. A heat transfer pipe as defined in claim 1, wherein thegrooves are U-shaped in section.
 8. A heat transfer pipe as defined inclaim 7, wherein the ratio between the depth and the width of groove is0.4 at minimum.
 9. A heat transfer pipe as defined in claim 7, whereinthe ratio between the depth and the width of groove is 4.0 at maximum.10. A heat transfer pipe adapted for use in a heat exchanger of an airconditioner, freezer, air separator, etc., which heat transfer pipe hasan inner diameter in the range between 5 and 20 millimeters and isadapted to permit a boiling liquid or a condensing liquid to flowtherethrough, said heat transfer pipe being formed on its inner wallsurface with grooves having a depth from the wall surface to theirbottoms in the range between 0.02 and 0.2 millimeters, a pitch betweenthe adjacent grooves in the range between 0.1 and 0.5 millimeters, andan angle of inclination with respect to the axis of the heat transferpipe in the range between 4° and 15° or 165° and 176°.
 11. A heattransfer pipe as claimed in claim 9, wherein the angle of inclination ofsaid grooves with respect to the axis of the heat transfer pipe is 7°.